When a fault is detected in a T-carrier span (either T1 or T1C) a fault locate signal is placed on the digital line. This signal is a digital signal with bipolar violations, set in triplet (trio) placed with spacing so as to create a low frequency energy on the line to develop a predetermined signal (frequency). If the signal passes through the filter it is an indication that the regenerator associated with that filter is functioning properly. If no signal is passed it indicates that the regenerator, the cable or either of the two apparatus cases involved is defective and should be replaced.